1. Technical Field
The embodiments of the present disclosure are related to the field of semiconductor packaging, and in particular to molds for encapsulating semiconductor devices in molding compound.
2. Description of the Related Art
The most common form of final packaging for semiconductor devices is a molded polymer case. Such packaging is used, for example, in traditional dual in-line (DIP) packages, dual and quad flat packs (DFP, QFP), and many others.
FIG. 1 is a simplified perspective view of a portion of the bottom plate 100 of a two-plate mold for encapsulating semiconductor devices. The plate 100 includes cavities 102 and a runner system 104 configured to deliver molding compound to the cavities. The runner system comprises a sprue 106, runners 108, and gates 110. In addition to the bottom plate 100 shown, a top plate with corresponding cavities is positioned above the bottom plate in a molding press, which is configured to move the top and bottom plates relative to each other during operation. The plane where the mold separates, at the faces of the top and bottom plates, is referred to as the parting line, indicated in FIG. 1 at 112.
Typically, semiconductor chips are mounted to a lead frame, with chip contacts wire bonded to fingers of the lead frame. One or both of the top and bottom plates are provided with depressions—not shown in FIG. 1—that correspond to the shape of the lead frame so that when a lead frame is positioned on the bottom plate 100, the plates can fully close over the lead frame.
In operation, the lead frame is positioned on the bottom plate 100 with the semiconductor chips positioned in the cavities 102. The top plate is then lowered onto the bottom plate 100, and molding compound is injected into the runner system 104 via the sprue 106. Molding compound flows from the sprue 106 to the runners 108, where it is distributed to the various cavities. The molding compound enters the cavities 102 via the gates 110, and flows over and around the semiconductor chips and those portions of the lead frame that are inside the cavity. A vent, not shown in FIG. 1, is provided to let gas and excess molding compound escape from the cavities 102. The molding compound in the cavities 102 and runner system 104 is cured, after which the plates are separated and the lead frame, with new packages attached, is removed from the mold. The lead frame and runner system element are then trimmed from the packages, usually leaving portions of the lead frame, in the form of contact pins, extending from the packages.
The molding system described above is referred to as a side-gate mold, because the gates 110 are positioned at the sides of the cavities 102, on the parting line between the top and bottom plates.
While the process described above has for years been adequate for most molded packaging requirements, recent developments in semiconductor technology have been found to be less compatible with the traditional processes. In particular, as circuit density has increased, the pitch of the circuit pads on many semiconductor chips has been significantly reduced, as compared to devices of a few years ago. This means that the bonding wires used to connect the chips to the lead frame fingers are also closer together. A 40 μm pad pitch is not uncommon, with bonding wires having diameters of around 10 μm. When such a device is positioned in a side-gate mold, the molding compound causes “wire sweep” as it flows across the cavity, in which the bonding wires are swept sideways, resulting in damage and short circuits.
In response to the increasing failure rate associated with packaging of high-density devices in side-gate molds, some manufacturers have begun to produce some packages in top-gate molds. FIGS. 2A and 2B are diagrammatical views, in cross-section, of a simplified example of a top-gate mold 120 used to package semiconductor devices. The mold 120 includes a top plate 122, a middle plate 124, and a bottom plate 126, configured to separate at top and bottom parting lines 123, 125. Cavities 128 are formed between the middle and bottom plates 124, 126, while a runner system 130 is formed between the top and middle plates 122, 124. The runner system 130 includes a sprue 132, a plurality of runners 134, reservoirs 136, reservoir locks 138, and gates 140. The top plate 122 also includes ejection pins 142. A portion of one runner 134 is shown in detail, but molds of the type shown can have many runners extending from a single frustoconically shaped sprue.
A lead frame 144 is also shown, with semiconductor chips 146 attached and positioned in the cavities 128. Wire bonds 148 couple the semiconductor chips 146 to lead frame fingers 150.
Typically, during operation, the top plate 122 remains stationary, while the middle and bottom plates 124, 126 move during each cycle. At the beginning of a cycle, a lead frame 144 is positioned on the bottom plate 126, which is then moved up against the middle plate 124, and both plates are moved up against the top plate 122. Molding compound is heated and introduced into the sprue 132 at a sprue bushing 133. The molding compound flows into the runners 134 and fills each reservoir 136 in turn. As the molding compound is filling the reservoirs, fluid pressure is relatively low, so very little of the viscous molding compound may pass the narrow gates 140. However, when the last reservoir is filled, fluid pressure increases, forcing the compound through the gates 140 and into the cavities 128. As molding compound flows into the runner system 130 and cavities 128, displaced air escapes through passages provided for that purpose, or via thin gaps between the plates. Because it flows into the cavities 128 from above the wire bonds 148, there is less of a tendency for the molding compound to sweep the wires out of position, so the failure rate is reduced, as compared to a side-gate mold system.
Once the cavities are filled, the molding compound in the cavities 128 and runner system 130 is cured, and the press is opened to remove the packages 129, as shown in FIG. 2B. First, the middle and bottom plates 124, 126 are separated from the top plate 122 at the first parting line 123. Most of the spaces in the mold 120 in which molding compound will be cured are provided with a draft. In other words, they have a taper that permits easy removal of the finished element from the mold. However, the reservoir locks 138 have sides that are substantially straight, or even slightly undercut, so the material that has hardened within the reservoir locks resists removal. Consequently, the runner system element 152 remains attached to the top plate 122, and the reservoir elements 137 and runner elements 135 are pulled from the middle plate 124 as the top and middle plates separate. This causes the molding compound reside 141 in the gates 140 to break, separating the runner system element 152 from the packages 129. After the runner system element 152 is separated from the packages 129, the ejector pins 142 move downward, ejecting the material from the reservoir locks 138 and separating the runner system element from the top plate 122. The middle and bottom plates 124, 126 also separate, and the lead frame is removed. Frequently, ejector pins are also provided in the bottom plate 126 to permit easy removal of the packages 129.
In the field of injection and transfer molding, there are two main classes of materials used to formulate molding compounds: thermoplastics, and thermosets. Thermoplastics are materials that soften or liquefy when heated, and harden when cooled. Thermoplastic materials can be remelted over and over again. In contrast, thermosetting materials require a curing process to harden, and once cured, cannot be remelted. When heated, they will decompose before they reach a melting temperature. Thermosets use a number of curing processes, depending on the material. Some thermosetting material is cured by heat, or a combination of heat and pressure; others employ a catalyst, exposure to a selected radiation (e.g., UV light), etc.
Of the two classes of materials, thermosets typically can be formulated to be stiffer and more rigid than thermoplastics, having a much higher modulus of elasticity. This is an important consideration in selecting the material for a molding compound for packaging semiconductor devices, because a semiconductor chip is relatively fragile, resembling an extremely thin piece of glass. Likewise, the bond wire connections that couple the chip to the lead frame fingers are also fragile. Flexure of the encapsulating material can break the chip or wires, or cause the wire connections to separate. Thus, molding compounds used to package semiconductor devices are generally thermosetting compounds having a very high modulus, to protect the underlying structure. Other considerations in formulating the compound include coefficient of thermal expansion; thermal conductivity; resistance to water, acids, and other chemicals; opacity to a broad spectrum of light and to other forms of radiation; etc. The intended operating conditions of a particular device will determine which factors are most relevant or important.
With regard to the design of the mold, there are a number of aspects that should be noted. Pressure drop from the sprue to the gate should be smooth and steady, without sudden changes; the surfaces of the runners should be perfectly smooth, with no unnecessary obstructions or changes in shape; and all of the cavities should fill at the same time, and under approximately equal pressure. These aspects are interrelated, and so must be considered together, in designing a mold. For example, a sudden high pressure drop in a runner can slow fluid flow at that point, causing downstream cavities to fill slowly or incompletely. Molding compound passing an unintended high-pressure choke point in a runner can generate heat and begin to cure prematurely. Surface textures or protruding features can create resistance to flow, and therefore increase pressure drop at that point. Additionally, such features can interfere with removal of the runner system element once curing is complete. Thus, runners are generally as short, smooth, and featureless as possible. If one cavity fills before the others, it can experience a higher pressure than the others, or can be under pressure for a longer duration. Differences in pressure and duration in the cavities can affect the cure rate, so that one or more of the packages could be under-cured or over-cured, in cases where packages are intentionally removed from the mold before they are fully cured. Excessive pressure in a mold cavity can also damage a semiconductor device.
In a mold like the one described with reference to FIGS. 2A and 2B, while molding compound is flowing, fluid pressure in the runners will drop along the length of each runner, so that the cavities nearest to the sprue will tend to experience higher pressure, and to be filled first, at least until the entire system is filled. This can be balanced by careful control of the gate diameters and runner dimensions. For example, by making the first gate diameter smaller than that of the next gate, which in turn is smaller than that of the third gate in line, etc., pressure drop at the first gate will be greater than at the second, offsetting the higher fluid pressure in the reservoir, so that pressure within the cavities is substantially equal. Additionally, the rheology of the uncured compound can be selected so that, once a reservoir is full, and while downstream reservoirs are still filling, the stillness of the fluid as it sits in the reservoir, and the relatively low pressure, permit the fluid to transition to a more viscous state. In this state, the compound does not immediately flow through the small gate orifice. However, once the last reservoir is filled and fluid pressure increases, the increased fluid pressure causes the thickened compound to begin to pass through the gate. The increased pressure together with the fast movement of the fluid through the gate cause the fluid to transition to a more liquid state. In the more liquid state, the molding compound is less likely to damage the chip, and especially the bonding wires, as it flows around the device to fill the cavity.
It can be seen that proper design of a runner system of a mold can be very challenging, and that accounting for and calculating the effects of all of the factors involved can be difficult or impossible. Mold designers tend therefore to be very conservative, avoiding any unnecessary or questionable features, changes in shape, or obstructions. When a runner system does not function properly, it can be extremely difficult to locate the cause. Sometimes, a problem in one part of a system is caused by a miscalculation related to an entirely different part of the system, in an area that appears to be operating perfectly.